In recent years, great strides have been made in reducing air leakage in residential and to a lesser extent small commercial forced air duct systems. Several authorities have introduced low leakage limits for thermal distribution systems; for example, the State of California Energy Code for Buildings gives credit for systems that leak less than 6\% of the total air flow at 25 Pa. Practitioners have found that a significant barrier to meeting specifications like this is the air leakage of the furnace or air handler itself. Anecdotal evidence exists for the magnitude of the air leakage of furnaces and air handlers. The states of California and Florida include air leakage limits for the furnaces in their State Building Energy Codes. However, there is currently no standard test method for measuring this air leakage that could be used for uniform and reliable ratings. This paper presents the results of laboratory measurements air leakage testing of furnaces and air handlers. The results indicate that average air leakage is significant - confirming existing anecdotal evidence. Also, the air leakage has a wide range from furnace to furnace indicating that low levels of air leakage are already attainable with existing equipment and the rating for air leakage will be able to distinguish between good and poorly performing equipment. This paper will also discuss the development of a standard test procedure (ASHRAE Standard 193 "Method of Test for Determining the Airtightness of HVAC Equipment") that will be used by Federal, State and Local code authorities and efficiency programs as well as appliance standards, utility programs and a range of other applications.